Some changes in Kirigami.Dialog recently completely broke the dialog, we
apparently now have to manually set dimensions on children. Use
DatePopup from Kirigami Addons in order to reduce the code needed here.
The old UI was very confusing to use, I changed it so that:
- Dialogs are used for the time and date picker, with user confirmation for changes
- Tumblers are not used for the time picker, use the time picker component from kclock
- Removed seconds selector (since it was always wrong)
- Fixed the time zone selector (it didn't work before)
- Properly support 24 hour time in the time picker
Fixes https://invent.kde.org/plasma/plasma-mobile/-/issues/286
